The present invention relates to an apparatus for holding grid springs, enabling quick insertion of fuel rods into grid cells of a fuel assembly.
In general, in a pressurized water reactor, a fuel assembly is used. The fuel assembly comprises numerous elongated fuel rods arranged in parallel, predeterminedly spaced in relation to one another. The fuel assembly consists of a top nozzle and a bottom nozzle arranged in a spaced relation, a plurality of grids assembled with a plurality of straps and arranged in a spaced relation to one another between the nozzles, control-guide pipes and measuring-instrument guide pipes passing through grid cells of the grids, and numerous fuel rods passing through the grid cells and elastically supported by a spring formed in each grid.
In the foregoing, in assembling a fuel assembly such as the above, when the fuel rod is just inserted in the grid cell of the grid without any supplementary care, the fuel rod contacts the spring and a dimple which is formed on the strap to support the fuel rod. The contact of the fuel rod with the spring and the dimple results in the occurrence of scratching on the peripheral outer surface of the fuel rod or damage to the grid. Therefore, up to now, a number of countermeasures have been suggested to overcome the above problems. As an example of a countermeasure to prevent occurrence of scratching, a grid having an opening for inserting a key is disclosed in Japanese Patent First Publication No. Hei 2-181699. In the apparatus, the fuel rod is inserted in the grid in such a way that a bar-shaped long key is inserted in a row of grid cells through the opening from the outside, then springs are pressed and reflected by the key. In such a fuel assembly, since the space of the grid cell can be enlarged to be greater than the diameter of the fuel rod, the fuel rod can be smoothly inserted without scratching on the peripheral outer surface of the fuel rod.
However, in the grid adapted to insert the key, the operations for inserting the key in a row of grid cells and pressing the springs are very complicated, and hence, a large amount of time is required to operate the enlargement for numerous grids. Therefore, a new solution in which springs can be easily and quickly reflected, so that numerous grid cells can be enlarged, is desired.